Following traumatic or mechanically induced axonal degeneration in the peripheral nervous system, axonal regeneration ensues, resulting in functional recovery. However, the rate of axonal elongation (3-4 mm/day) is slow. Consequently, recovery is measured in weeks or months, depending upon the distance between the site of injury and the target tissue. Therapies that speed regeneration over long distances would be highly beneficial to patients and would significantly reduce health care costs.
The immunosuppressant drug FK506 (USAN tacrolimus; Prograf(copyright)) speeds functional recovery and axonal regeneration in the rat in a dose-dependent manner following a sciatic nerve crush lesion (Gold et al., J. Neurosci. 15:7505-7516, 1995; Gold et al., Restor. Neurol. Neurosci. 6:287-296, 1994). FK506 was shown to stimulate neuritic outgrowth in a rat pheochromocytoma cell line in a concentration-dependent manner (Lyons et al., Proc. Natl. Acad. Sci. USA 91:3191-3195, 1994).
Systemic administration of two synthetic FK506 analogs that bind FKBP-12 but that do not inhibit calcineurin activity (and which are not immunosuppressants) increases the size of myelinated fibers (Steiner et al., Nature Medicine 3:1-8, 1997; Steiner et al., Proc. Natl. Acad. Sci. USA 94:2019-2024, 1997). U.S. Pat. No. 5,654,332 (Armistead et al.) discusses immunosuppressive FK506 analogs that bind FKBP12 and that are said to stimulate neurite outgrowth in the presence of NGF. It was stated that the neurotrophic activity of these FKBP12 binding compounds xe2x80x9cis directly related to their affinity for FKBP12 and their ability to inhibit FKBP12 rotomase activityxe2x80x9d (id. at col. 7, lines 47-50).
It has been reported that androgens and estrogens stimulate facial nerve regeneration in hamsters (Jones, xe2x80x9cAndrogenic enhancement of motor neuron regeneration,xe2x80x9d In: Luine and Harding, eds., Hormonal Restructuring of the Adult Brain, Ann. N.Y. Acad. Sci. 85:141-164, 1994; Tanzer and Jones, Exp. Neurol. 146:258-264, 1997).
I have discovered that geldanamycin and FK506 stimulate nerve regeneration via a common mechanism. Both compounds bind to polypeptide components of steroid receptor complexes, hsp90 and FKBP52, respectively. These and other compounds that cause hsp90 dissociation from steroid receptor complexes or that block association of hsp90 with steroid receptor complexes stimulate nerve cell growth and promote nerve regeneration. Such compounds can act directly by binding to hsp90 (as in the case of geldanamycin) or indirectly by binding to another polypeptide in the steroid receptor complex (as in the case of FK506 binding of FKBP52).
According to one aspect of the invention, pharmaceutical compositions are provided that include a nerve growth stimulating amount of a non-FKBP12-binding agent that binds to a polypeptide component of a steroid receptor complex other than the ligand (i.e., steroid hormone) binding portion thereof (such polypeptide components including, but not limited to, hsp90 or FKBP52) and a pharmaceutically acceptable excipient. Without limitation to any particular mechanism of action, binding of such agents to the polypeptide component likely causes hsp90 dissociation from the complex or prevents hsp90 association with the complex. Nerve growth promoting agents according to the invention include, but are not limited to non-FKBP12-binding FK506 analogs, benzoquinone ansamycins (e.g., geldanamycin and derivatives thereof), peptides that comprise a sequence of a selected polypeptide component of the complex at a site of interaction between the selected component and another component of the complex, antibodies that bind a polypeptide component of the steroid receptor complex, and combinations thereof.
According to another aspect of the invention, such pharmaceutical compositions include other active ingredients, including, but not limited to, neurotrophic factor other than the nerve growth promoting agent (e.g., NGF, IGF-1, aFGF, bFGF, PDGF, BDNF, CNTF, GDNF, NT-3, NT 4/5, or mixtures thereof), and a steroid ligand of the steroid receptor complex (e.g., estrogen and dexamethasone, as in the Examples below).
According to another aspect of the invention, a transection (severing of the nerve) of a peripheral nerve or a spinal cord injury of a mammal is treated by methods that include administering a nerve growth stimulating amount of a non-FKBP12-binding nerve growth promoting agent to the mammal and grafting to the peripheral nerve or spinal cord an allograft or an artificial nerve graft. In the case of a transected peripheral nerve or spinal cord, the space between the transected ends of the peripheral nerve or spinal cord is preferably filled with a material such as collagen, methyl cellulose, or a cell suspension that promotes nerve cell growth, such as Schwann cells and olfactory and sheathing cells. The nerve growth promoting agent can be included together with such filling materials.
According to another aspect of the invention, pharmaceutical compositions are provided that include a nerve growth stimulating amount of a non-FKBP12-binding FK506 analog that binds to FKBP52 and a pharmaceutically suitable excipient.
According to another aspect of the invention, pharmaceutical compositions are provided that include a nerve growth stimulating amount of an agent that binds to hsp90 and causes hsp90 dissociation from a steroid receptor complex or prevents hsp90 association with the complex and a pharmaceutically suitable excipient.
According to another aspect of the invention, methods of stimulating nerve cell growth in a mammal are provided that include administering a pharmaceutical composition as described herein.
According to another aspect of the invention, methods are provided for stimulating growth of a nerve cell that include contacting the nerve cell with a non-FKBP12-binding agent that binds to a polypeptide component of a steroid receptor complex other than the steroid hormone binding portion thereof and causes hsp90 dissociation from the complex or blocks association of hsp90 with the complex.
According to another aspect of the invention, methods of identifying compounds that stimulate nerve cell growth are provided that include the steps of assaying test compounds for binding to a component of a steroid receptor complex other than the steroid hormone binding portion thereof, and assaying the binding compounds for stimulation of nerve cell growth. In addition, the binding compounds can be assayed for activity in dissociation of hsp90 from the complex or blocking association of hsp90 with the complex.
The foregoing and various features and advantages of the invention will become more apparent from the following detailed description and accompanying drawings.